Evidence for Nanoflare Heating of the Solar Corona from the EUNIS Sounding Rocket

Abstract:

We present spatially resolved EUV spectroscopic measurements of
pervasive, faint Fe XIX 592.2 A line emission in AR 11726,
observed during the 2013 April 23 flight of the Extreme Ultraviolet
Normal Incidence Spectrograph (EUNIS-13) sounding rocket instrument.
With cooled detectors, high sensitivity, and high spectral
resolution, EUNIS-13 resolves the lines of Fe XIX at 592.2 A (formed
at temperature T around 8.9 MK) and Fe XII at 592.6 A (T around 1.6
MK). The Fe XIX line emission, observed over an area in excess of
4920 square arcsec (2.58x10^9 square km, more than 60% of the active
region), provides strong evidence for the nanoflare heating model of
the solar corona. No GOES events occurred in the region less than 2
hours before the rocket flight, but a microflare was observed north
and east of the region with RHESSI and EUNIS during the flight. The
absence of significant upward velocities anywhere in the region,
particularly the microflare, indicates that the pervasive Fe XIX
emission is not propelled outward from the microflare site, but is
most likely attributed to localized heating (due to reconnection,
wave dissipation, or some other mechanism) consistent with the
nanoflare heating model of the solar corona. We measure average Fe
XIX/Fe XII intensity ratios of 0.070 outside the AR core, 0.22 in
area of bright coronal emission (the area in which the Fe XII
intensity exceeds half its maximum observed value), and 0.55 in the
region's hot core. Using the CHIANTI atomic physics database and
assuming ionization equilibrium, we estimate corresponding Fe XIX/Fe
XII emission measure ratios of about 0.076, 0.23 and 0.59. The
emission measure ratios must be viewed with caution in light of
lingering uncertainties in the Fe XII contribution functions.
EUNIS-13 was supported by the NASA Heliophysics Division through its
Low Cost Access to Space program.